671 research outputs found
Nonlinear screening in two-dimensional electron gases
We have performed self-consistent calculations of the nonlinear screening of
a point charge Z in a two-dimensional electron gas using a density functional
theory method. We find that the screened potential for a Z=1 charge supports a
bound state even in the high density limit where one might expect perturbation
theory to apply. To explain this behaviour, we prove a theorem to show that the
results of linear response theory are in fact correct even though bound states
exist.Comment: 4 pages, 4 figure
The role of surface plasmons in the decay of image-potential states on silver surfaces
The combined effect of single-particle and collective surface excitations in
the decay of image-potential states on Ag surfaces is investigated, and the
origin of the long-standing discrepancy between experimental measurements and
previous theoretical predictions for the lifetime of these states is
elucidated. Although surface-plasmon excitation had been expected to reduce the
image-state lifetime, we demonstrate that the subtle combination of the spatial
variation of s-d polarization in Ag and the characteristic non-locality of
many-electron interactions near the surface yields surprisingly long
image-state lifetimes, in agreement with experiment.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Nonlinear screening and stopping power in two-dimensional electron gases
We have used density functional theory to study the nonlinear screening
properties of a two-dimensional (2D) electron gas. In particular, we consider
the screening of an external static point charge of magnitude Z as a function
of the distance of the charge from the plane of the gas. The self-consistent
screening potentials are then used to determine the 2D stopping power in the
low velocity limit based on the momentum transfer cross-section. Calculations
as a function of Z establish the limits of validity of linear and quadratic
response theory calculations, and show that nonlinear screening theory already
provides significant corrections in the case of protons. In contrast to the 3D
situation, we find that the nonlinearly screened potential supports a bound
state even in the high density limit. This behaviour is elucidated with the
derivation of a high density screening theorem which proves that the screening
charge can be calculated perturbatively in the high density limit for arbitrary
dimensions. However, the theorem has particularly interesting implications in
2D where, contrary to expectations, we find that perturbation theory remains
valid even when the perturbing potential supports bound states.Comment: 23 pages, 15 figures in RevTeX
Spin-helical Dirac states in graphene induced by polar-substrate surfaces with giant spin-orbit interaction: a new platform for spintronics
Spintronics, or spin electronics, is aimed at efficient control and
manipulation of spin degrees of freedom in electron systems. To comply with
demands of nowaday spintronics, the studies of electron systems hosting giant
spin-orbit-split electron states have become one of the most important
directions providing us with a basis for desirable spintronics devices. In
construction of such devices, it is also tempting to involve graphene, which
has attracted great attention because of its unique and remarkable electronic
properties and was recognized as a viable replacement for silicon in
electronics. In this case, a challenging goal is to make graphene Dirac states
spin-polarized. Here, we report on absolutely new promising pathway to create
spin-polarized Dirac states based on coupling of graphene and polar-substrate
surface states with giant Rashba-type spin-splitting. We demonstrate how the
spin-helical Dirac states are formed in graphene deposited on the surface of
BiTeCl. This coupling induces spin separation of the originally spin-degenerate
graphene states and results in fully helical in-plane spin polarization of the
Dirac electrons.Comment: 5 pages, 3 figure
Inelastic lifetimes of hot electrons in real metals
We report a first-principles description of inelastic lifetimes of excited
electrons in real Cu and Al, which we compute, within the GW approximation of
many-body theory, from the knowledge of the self-energy of the excited
quasiparticle. Our full band-structure calculations indicate that actual
lifetimes are the result of a delicate balance between localization, density of
states, screening, and Fermi-surface topology. A major contribution from
-electrons participating in the screening of electron-electron interactions
yields lifetimes of excited electrons in copper that are larger than those of
electrons in a free-electron gas with the electron density equal to that of
valence () electrons. In aluminum, a simple metal with no -bands,
splitting of the band structure over the Fermi level results in electron
lifetimes that are smaller than those of electrons in a free-electron gas.Comment: 4 papes, 2 figures, to appear in Phys. Rev. Let
Direct resolution of unoccupied states in solids via two photon photoemission
Non-linear effects in photoemission are shown to open a new access to the
band structure of unoccupied states in solids, totally different from hitherto
used photoemission spectroscopy. Despite its second-order nature, strong
resonant transitions occur, obeying exact selection rules of energy, crystal
symmetry, and momentum. Ab-initio calculations are used to demonstrate that
such structures are present in low-energy laser spectroscopy experimental
measurements on Si previously published. Similar resonances are expected in
ultraviolet angle-resolved photoemission spectra, as shown in a model
calculation on Al.Comment: 12 pages, including 4 figure
Ultrafast electron dynamics in metals
During the last decade, significant progress has been achieved in the rapidly
growing field of the dynamics of {\it hot} carriers in metals. Here we present
an overview of the recent achievements in the theoretical understanding of
electron dynamics in metals, and focus on the theoretical description of the
inelastic lifetime of excited hot electrons. We outline theoretical
formulations of the hot-electron lifetime that is originated in the inelastic
scattering of the excited {\it quasiparticle} with occupied states below the
Fermi level of the solid. {\it First-principles} many-body calculations are
reviewed. Related work and future directions are also addressed.Comment: 17 pages, two columns, 13 figures, to appear in ChemPhysChe
An exact expression to calculate the derivatives of position-dependent observables in molecular simulations with flexible constraints
In this work, we introduce an algorithm to compute the derivatives of
physical observables along the constrained subspace when flexible constraints
are imposed on the system (i.e., constraints in which the hard coordinates are
fixed to configuration-dependent values). The presented scheme is exact, it
does not contain any tunable parameter, and it only requires the calculation
and inversion of a sub-block of the Hessian matrix of second derivatives of the
function through which the constraints are defined. We also present a practical
application to the case in which the sought observables are the Euclidean
coordinates of complex molecular systems, and the function whose minimization
defines the constraints is the potential energy. Finally, and in order to
validate the method, which, as far as we are aware, is the first of its kind in
the literature, we compare it to the natural and straightforward
finite-differences approach in three molecules of biological relevance:
methanol, N-methyl-acetamide and a tri-glycine peptideComment: 13 pages, 8 figures, published versio
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